There are many different applications in which geophones are placed below the earth's surface, down a well or other borehole. In many of these applications it is necessary to know the orientation of the geophones as accurately as possible. This invention provides a method and apparatus for accurately determining the position and orientation of geophones once they have been positioned below ground.
Where the geophones are placed in shallow holes, the orientation and placement of the geophones may be determined from visual inspection, or it may be possible to place the geophones in the hole in such a way that the orientation and position is predetermined. However, in the more common case where the geophones are placed down a deep hole, there are considerable problems in accurately determining exactly where the geophones are located and how they are orientated after they have been lowered into the well or borehole. Even if the geophones are tightly attached to the tubing string, the tubing has a tendency to twist and deviate from the vertical as it is descending through the hole.
Commonly, the prior art has attempted to solve this problem by analyzing signals received by geophones from seismic sources with known locations relative to the hole in which the geophones are located. However, this analysis requires a detailed knowledge of the seismic characteristics of the earth between the source and the geophones in order to give an accurate answer, and this knowledge is often incomplete or uncertain. Further, the process of obtaining and analyzing the data is expensive and time-consuming.
Other prior art has used oriented devices which are an integral part of the geophone housing such as remote reading compasses. This method provides high accuracy but will not operate in proximity to iron. This excludes application in the vicinity of steel tubing or inside steel casing. Steel tubing is a very effective way of loading a cable with geophones into a borehole, and to introduce cement into the hole. Some applications for the downhole geophones call for use inside steel casing.
There are additional difficulties associated with the compass. Package design is considerably more costly due to length required to separate the compass from magnetic geophones. This length decreases the ability of the package to withstand pressures found deeper in boreholes. Remote reading devices also increase the requirements for electrical conductors in the cable and for surface operation and measurement control. Typically, several pairs of conductors would be required for each level.